Most modern point-to-point optical communication links require that nodes communicate in both directions, as shown in FIG. 1a. In this example, a bidirectional communication installation 12 facilitates bidirectional communication between a first transceiver 21a and a second transceiver 21b. A first discrete optical fiber 36a carries a signal from a first transmitter 24a to a first receiver 27a, and a second discrete optical fiber 36b carries a signal in the opposite direction from a second transmitter 24b to a second receiver 27b. To communicate between the first transceiver 21a and the second transceiver 21b, each of the optical cables 36a, 36b require optical connectors 42 for each bulkhead 45 they pass through.
In general, optical connectors 42, contacts within the connectors, and cable represent a substantial part of the cost of a fiber optic link (typically greater than 50% of link cost). They also constitute the least reliable components within the link, in part because connectors are intended to be de-mated, allowing contamination, and in part, because these components (in particular the cables) are exposed to maintenance-induced failure due to their accessibility within equipment bays, etc. Thus, any scheme that allows both signal paths, the incoming signal 33 and the outgoing signal 30 (FIG. 1a), to be combined over a single fiber substantially reduces cost and improves reliability (e.g. there are half as many connectors and half as many cable segments to go bad). There are several techniques that have been used to accomplish this bidirectional transmission, each with disadvantages.
A first alternate installation scheme 15 is to use an optical coupler 36f to combine the signals 30, 33 on a single optical fiber 36 strung between the first transceiver 21a and the second transceiver 21b, as shown in FIG. 1b. The optical coupler 36f in the first alternate installation scheme 15 may be a fused bi-conic tapered coupler with the cores of single optical fiber 36 and the second optical fiber 36d in intimate contact with one another. One object of the first alternate installation 15 is accomplished in that the number of optical connectors 42 through the bulkheads 45 is decreased by a factor of two.
One disadvantage of this first alternate installation 15 is that half of the light from the transmitter (assuming a 50–50 coupler) is lost before it makes its way to the single optical fiber 36 through the bulkheads 45 at the connectors 42 because half the light is coupled into an unused fourth port 36e. There is a further loss at the other receiving end of the single optical fiber 36, where half the light is coupled into the transmitter 24b rather than the receiver 27a. As a result, a theoretically perfect link will exhibit a 6dB loss from transmitter 24a to receiver 27a just because of these couplers 36f. It is desirable in most links to avoid this very large loss.
A second alternate installation 18 includes use of two discrete wavelengths 51, 54, as shown in FIG. 1c. The use of dichroic (two-color) filters 57a, 57b (sometimes called dichroic mirrors) is just one of many possible approaches but is the most straightforward schematically and is used here to describe the general approach. The transmitter 24a at the first transceiver 21 a creates light 51 with wavelength of λ1. The filter at this node transmits light 51 at λ1 but reflects light 54 at λ2. Incoming light 54 carrying an incoming signal 33 with wavelength λ2 is routed to the receiver 27b along the fiber 37. Similarly, the right-hand node dichroic filter 57b transmits light 54 at wavelength λ2 carrying an outgoing signal 30 but reflects light 51 at wavelength λ1. Alternatively, fibers 30 and 37 may be replaced by free space illumination from the transmitter 24a and to the receiver 27b without affecting this example.
There are several disadvantages to the second alternate installation 18. Principal among these is the fact that light 54 returning from reflections on the single optical fiber 36 is transmitted entirely back to the transmitter 24b of origin. This may be disadvantageous as it can cause laser instability. In addition, the transceivers 21a, 21b at the two ends of the link are unique from one another. In FIG. 1c, the transmitter 24a on the left-hand side emits light 51 at wavelength λ1 but the transmitter 24b on the right-hand side emits light 54 at wavelength λ2. Likewise, the receivers 27a, 27b operate at different wavelengths. Finally, as systems increasingly make use of multiple wavelengths, it is important to keep in mind that this scheme halves the number of wavelengths available for information carrying. Notably, although this approach is conceptually simple, dichroic filters 57a, 57b are costly and that cost may drive the cost of the installation.
Therefore, a need exists for bidirectional optical signal transmission apparatus and methods that at least partially mitigate the above-noted disadvantages in an economical manner.